I'm only guessing here. It sounds like you tackle big, well-known pieces; your enthusiasm for them propels you to make pretty good initial progress, and that's sort of satisfying. But I'm guessing that by the time you stop, something inside is saying something like "Damn, that was fun, but it's gonna take an awful lot of work to really finish it." That gets discouraging, and the motivation wanes. That happens many times and you get into a pattern of feeling like you can never finish anything, so once the initial burst of enthusiasm dies away, it's hard to get motivated to continue.My suggestion is that you try to find shorter, easier pieces that motivate you musically. Unless your previous motivation has all come from a desire to thunder out big virtuoso pieces, then you should be able to find some beautiful pieces that you love and that you can actually work up to a good standard in a relatively short time. Hopefully, the satisfaction of getting pieces solid will build up your motivation for regular practice over time.Also, just listen to lots of music, other than the big virtuoso monsters of the piano repertoire. Just enjoy it. That should keep the motivation going, too.